What is Docker, and why is it used?

Docker is a containerization platform that allows developers to package applications along with their dependencies into a single unit called a container.

• Why use Docker?
  ✅ Ensures consistent environments across different machines.
  ✅ Lightweight & faster than virtual machines.
  ✅ Easy scaling of applications in microservices architectures.

What is the difference between Docker and a Virtual Machine (VM)?

What is a Docker image?

A Docker image is a read-only template containing everything needed to run an application, including:

• Source code
• Libraries & dependencies
• Configuration files

A container is created from a Docker image using the docker run command.

What is a Docker container?

A Docker container is a running instance of a Docker image. It is:
✅ Lightweight (shares OS kernel)
✅ Isolated (has its own filesystem, network, and process space)
✅ Portable (can run on any system with Docker installed)

How do you create and run a Docker container?

To run a container from an image:

docker run -d --name myapp nginx

• -d: Run in detached mode (background).
• --name myapp: Name the container myapp.
• nginx: Use the nginx image.

What is the purpose of the Dockerfile?

A Dockerfile is a script that contains instructions to build a Docker image.
Example Dockerfile:

FROM node:16
WORKDIR /app
COPY . .
RUN npm install
CMD ["node", "app.js"]

• FROM: Base image.
• WORKDIR: Set working directory.
• COPY: Copy files.
• RUN: Execute commands (install dependencies).
• CMD: Define the default command to run.

What are Docker volumes?

Docker volumes store persistent data outside a container's filesystem.

• Types:
  • Anonymous Volumes: docker run -v /data nginx
  • Named Volumes: docker volume create mydata
  • Bind Mounts: docker run -v /host/path:/container/path nginx

How do you list running Docker containers?

Use the command:

docker ps

To list all containers (including stopped ones):

docker ps -a

What is Docker Compose?

Docker Compose is a tool for defining and running multi-container applications.

• Example docker-compose.yml:

  version: "3"
  services:
    web:
      image: nginx
      ports:
        - "80:80"
    db:
      image: mysql
      environment:
        MYSQL_ROOT_PASSWORD: root

• Start with: docker-compose up -d

• Stop with: docker-compose down

What is the difference between CMD and ENTRYPOINT in Docker?

What is Kubernetes?

Kubernetes (K8s) is an orchestration platform for managing containerized applications.

• Features:
  ✅ Automated scaling
  ✅ Self-healing (restarts failed containers)
  ✅ Load balancing
  ✅ Rolling updates

What is a Kubernetes Pod?

A Pod is the smallest unit in Kubernetes. It groups one or more containers that share the same network and storage.

What is a Kubernetes Deployment?

A Deployment manages Pod creation and updates.
Example YAML:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app
spec:
  replicas: 3
  selector:
    matchLabels:
      app: my-app
  template:
    metadata:
      labels:
        app: my-app
    spec:
      containers:
        - name: app
          image: nginx

• replicas: 3 → Runs 3 instances.
• matchLabels → Ensures the correct Pods are managed.

What is a Kubernetes Service?

A Service exposes a set of Pods over a network.

• Types:
  • ClusterIP (default)
  • NodePort (exposes on a fixed port)
  • LoadBalancer (uses cloud provider's load balancer)